Protective effects of dexmedetomidine in vital organ injury: crucial roles of autophagy

Vital organ injury is one of the leading causes of global deaths. Accumulating studies have demonstrated that dexmedetomidine (DEX) has an outstanding protective effect on multiple organs for its antiinflammatory and antiapoptotic properties, while the underlying molecular mechanism is not clearly understood. Autophagy, an adaptive catabolic process, has been found to play a crucial role in the organ-protective effects of DEX. Herein, we present a first attempt to summarize all the evidence on the proposed roles of autophagy in the action of DEX protecting against vital organ injuries via a comprehensive review. We found that most of the relevant studies (17/24, 71%) demonstrated that the modulation of autophagy was inhibited under the treatment of DEX on vital organ injuries (e.g. brain, heart, kidney, and lung), but several studies suggested that the level of autophagy was dramatically increased after administration of DEX. Albeit not fully elucidated, the underlying mechanisms governing the roles of autophagy involve the antiapoptotic properties, inhibiting inflammatory response, removing damaged mitochondria, and reducing oxidative stress, which might be facilitated by the interaction with multiple associated genes (i.e., hypoxia inducible factor-1α, p62, caspase-3, heat shock 70 kDa protein, and microRNAs) and signaling cascades (i.e., mammalian target of rapamycin, nuclear factor-kappa B, and c-Jun N-terminal kinases pathway). The authors conclude that DEX hints at a promising strategy in the management of vital organ injuries, while autophagy is crucially involved in the protective effect of DEX.

Pharmacokinetic properties of DEX 4-[(1S)-1-(2,3-dimethylphenyl)ethyl]-1H-imidazole (DEX) is the dextro-enantiomer of medetomidine, with molecular formula C 13 H 16 N 2 (molecular mass 236.7 g/mol; octanol/water partition coefficient 2.89) [31,32]. DEX is currently approved for intravenous use, while the loading doses and infusion rates are based on a milligram per kilogram total body weight. DEX shows high protein binding (94% is bound to albumin and α1-glycoprotein) with an extensive volume of distribution (1.31-2.46 L/kg) and simply crosses the blood-brain barrier [32]. The elimination half-life of DEX in the adult health population and ICU patients is 2.1-3.1 h and 2.2-3.7 h, respectively [32,33], while the metabolic clearance in adult patients and ICU patients is 36-42 l/h and 31.8-57 l/h, respectively [33,34]. In children, the elimination half-life of DEX is approximate 2 h [35]. The pharmacodynamics of DEX includes sedative and hypnotic effects, analgesic effects, cardiovascular effects, respiratory effects, etc. The sedative and hypnotic effects developed by DEX may be associated with activation of central presynaptic and postsynaptic alpha 2 adrenoceptor in the locus coeruleus, regulation of endogenous sleeppromoting pathways, and an impact on the γ-aminobutyric acid system [32]. Significant and rousable sedation effects induced by DEX are recorded at plasma concentrations between 0.2 and 0.3 ng/mL. The analgesic effects of DEX are thought to be mediated through alteration of perception and reduction of anxiety. DEX has a biphasic hemodynamic effect on the cardiovascular system, showing that low plasma concentrations induce hypotension whereas higher concentrations lead to pulmonary and systemic hypertension [36]. As reported, the hypertensive effects of DEX overcome the hypotensive effects at concentrations between 1.9 and 3.2 ng/mL [36]. Minimal respiratory depression is observed at therapeutic plasma concentrations up to 2.4 ng/mL, showing a preservation of ventilatory response to CO 2 [37]. The ventilatory frequency can elevate with dose escalation of DEX, which compensates for slightly decreased tidal volumes [32]. With target concentrations between 0.2 and 0.6 ng/mL of DEX, no relevant pharmacokinetic interactions were identified in DEX when combining with propofol, isoflurane, midazolam, or alfentanil [32].

Current knowledge
α2-Receptors are frequently detected in various vital organs, including the central nervous system, kidneys, lungs, and liver [32]. Since DEX is a highly selective α2 adrenoceptor agonist, it may mediate a broad spectrum of pharmacodynamic actions on these organs. In numerous animal studies [18,38,39], DEX appears to alleviate the inflammation responses and the I/R injury of multiple organs, i.e., the brain, liver, and intestines. More importantly, although α2 adrenoceptor is not found in the myocardium, a large body of previous studies suggest that DEX plays a protective role on myocardial I/R injury [40,41]. DEX-mediated modulation of autophagy is considered to play the adrenergic receptor agonist's protective role in multiple organ injuries. Based on the above evidence, DEX exerts an encouraging protective effect on multiple organs. Mechanistically, recent experimental research has suggested that autophagy might be involved in this action. In this review, we thus outline the molecular and biological functions of autophagy in DEX-mediated organ-protective effects.

Cerebral injury
Fourteen publications reported autophagy involving the action of the protective effect of DEX in brain injury. The experimental models among these eligible studies included rat, mouse, and neurocyte (i.e., astrocytes, PC12, and neuroblastoma cells). The types of central nervous injury included cerebral ischemia/reperfusion injury, traumatic brain injury, neurological injury, cognitive impairment, hippocampus injury, oxygen-glucose deprivation-reoxygenation injury, and neonatal hypoxic ischemia. The route for DEX administration in an animal model included intraperitoneal injection and intravenous injection via the femoral vein or the caudal vein. The dose of DEX in a rat model ranged from 3 to 50 μg/kg, but 20-25 μg/kg in a mouse model. DEX in a cell model was administrated through cell supernatants, while the dose of DEX was 1 μM. Most of the included studies (12/14, 86%) reported the status of autophagy was inhibition in the protective effect of DEX in cerebral injury. Multiple genes and signaling pathways have been found to be involved in autophagy-mediated neuroprotection by DEX.
The characteristics and the main findings of the 14 relevant studies reporting cerebral injury are summarized in Table 1. Figure 1 shows the main mechanisms of autophagy in the cerebra-protective effects of DEX.

mTOR signaling pathway
The phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway is one of the most important signaling pathways with a critical biological function in various diseases [60,61], including neurological disorders [62]. As reported, numerous drugs exert their neuroprotective effect via the PI3K/Akt/mTOR signaling pathway [63]. mTOR is considered to serve as a central player in the regulation of autophagy because it can inhibit autophagy in the process of growth factors and abundant nutrients [64]. Shen et al. [42] demonstrated that DEX alleviated the degree of traumatic brain injury via inhibition of neuronic autophagy by activating the PI3K/AKT/ mTOR signaling pathway. In line with this finding, some investigators also found that the inhibition of neuronic autophagy was one of the therapeutic targets for traumatic brain injury treatment [65]. Also, the protective effects of DEX are speculated to be against the process of autophagy and apoptosis. Zhu et al. [48] reported that DEX increased the viability and inhibits apoptosis of astrocytes exposed to oxygen-glucose deprivation, which might be related to the activation of autophagy by regulating the tuberous   14%) reporting that the status of autophagy is activation when treating with DEX for cerebral injury. Commonly, autophagy is activated in cerebral injury [66], while DEX may inhibit the autophagy level and thus contribute to the neuroprotection in cerebral damage [53]. With the same cell line of astrocytes as used in Zhu et al. 's study [48], Qin et al. [67] suggested that the inhibition of autophagy might exert the protective effect on astrocytes after ischemic astrocyte injury. This could be partially explained by the finding that autophagy may play different roles in different cerebral injury stages, i.e., ischemia and reperfusion [48]. The exact roles of autophagy at different timepoints after cerebral injury deserve further investigation.

MicroRNAs (miRNAs)
miRNAs are a major class of conserved short noncoding RNAs with crucial biological functions in the regulation of a third of the whole genome at the posttranslational level [68]. miRNAs exert their roles by increasing messenger RNA degradation or by blocking messenger RNA translation [69]. Numerous studies have suggested that there is a close association between miRNAs and autophagy in various diseases, including cerebral injury [70]. Also, miRNAs-mediated autophagy and the signaling cascades might play critical roles in the effect of DEX in protecting cerebral injury. Li et al. [50] showed that DEX could improve the neurological outcome in a traumatic brain injury rat model by inhibiting autophagy and regulating the circLrp1b/miR-27a-3p/Dram2 pathway. They found that the protective effect of DEX after cerebral injury might be attributable to the downregulation of circLrp1b and the inhibition of injury-induced autophagy, while these effects were dramatically abolished by miR-27a-3p suppression. Zhu et al. reported that the autophagy level in the cerebral cortex increased in an animal model of cerebral ischemia/reperfusion injury, while inhibited autophagy was observed after treating with DEX. During this action, Zhu et al. further found that DEX significantly inhibited the expression of miR-199a and thus improved neurocyte injury. The above evidence indicated that inhibition of autophagy might be involved in the DEX-induced neuroprotective effect in cerebral injury.

Autophagy-associated proteins (Beclin-1, Bcl-2, LC3-I, and LC3-II)
The therapeutic implications of DEX in brain injury may also be strongly associated with the altered expression of autophagy-associated proteins such as Beclin-1, Bcl-2, LC3-I, and LC3-II. Beclin-1 is involved in the initiation and maturation steps of autophagy, constituting the primary component of the autophagy mechanism [71]. Bcl-2, one of the key interacting proteins of Beclin-1 and the antiapoptotic family members, can suppress autophagy initiation by inhibiting the cascade of autophagy formation [72]. Both LC3-I and LC3-II are biomarkers for autophagy. LC3B-II/I indicates the generation of autophagosomes. Shan et al. [43] found that DEX improved the abnormal morphology  [51] showed that the protective effects of DEX were evidenced by the inhibition of excessive autophagy of neurons and microglia through downregulating LC3B-II and Beclin1.
In line with Xue et al. 's findings, Yi et al. [44] found that the protective functioning developed by DEX might be via decreasing autophagy of hippocampal neurons, which presented with the reduction of LC3-I, LC3-II, and Beclin-1 expression. In contrast, though Yu et al. [21] detected that DEX attenuated hippocampus injury, they observed that the status of mitophagy was activated, characterized by enhancing LC3-II/LC3-I expression.

Myocardial injury
DEX has also been reported to elicit cardioprotective effects via various molecule mechanisms. Autophagy regulation is considered to be one of the proposed mechanisms, which is believed to constitute a crucial process in the self-preservation of the heart. As reported, autophagy machinery involves immunity modulation through transmitting microbes to lysosomes for degradation and facilitating the release of cytokines for microbe digestion [73]. To date, four experimental studies have confirmed the essential roles of autophagy in DEX-mediated cardioprotection [28,41,54,55] (Table 2). According to Yu et al. 's study, DEX could attenuate septic myocardial injury by increasing autophagic flux via activating α7nAChR and the Akt/GSK-3β cascades, resulting in a reduction of the myocardium apoptosis and inflammatory response. In line with this finding, Xiao et al. also found that autophagy upregulation was associated with the action that DEX protected human cardiomyocytes against I/R injury. They further observed that α2-adrenergic receptor/AMPK signaling cascades greatly contributed to the activation of autophagy during the protective process developed by DEX. Conversely, although two other studies [41,54] have also reported that DEX treatment significantly attenuated myocardium injury, the researchers found that the autophagy status was inhibited in this process. Zhang et al. [54] demonstrated that DEX alleviated myocardial ischemia/reperfusion injury by dramatically decreasing overautophagy and reducing cardiomyocyte apoptosis, oxidative stress, and inflammatory reactions via upregulating the SIRT1/mTOR axis. DEX postconditioning could result in a decrease    of LC3II and Beclin-1 and an elevation of p62 protein level, thus inhibiting autophagy. Li et al. [41] reported that DEX upregulated the phosphorylation of Beclin1 by activating the PI3K/Akt pathway and reduced the interactions of Atg14L-Beclin 1-Vps34 complex, thus inhibiting autophagy and protecting against myocardial ischemia/reperfusion injury. As shown in Table 2, DEX administration in both Zhang et al. and Li et al. 's study was based on intravenous injection, and the dose was the same at 10 μg/kg. The mechanisms of autophagy in the myocardial-protective effects of DEX are illustrated in Fig. 2.

Kidney injury
Acute kidney injury, a disease with high morbidity and mortality, is closely associated with multiple organ dysfunction. Kidney I/R injury and sepsis are the two main causes for the development of acute kidney injury. Autophagy has been shown to elicit some protective effects in the pathological processes of renal tubular injury [74]. A previous study [29] suggested that DEX preconditioning ameliorated kidney I/R injury and inflammatory response via the enhancement of autophagy and upregulation of the renal p38-CD44 pathway. The authors found that autophagy was markedly downregulated by kidney I/R injury, while intravenous treatment with 10 μg/kg DEX effectively prevented the impairment of the autophagic response, thus maintaining the degradation and recycling of multiple cellular components [29]. Consistent with this finding, two subsequent studies also observed that the renoprotective effects of DEX were mediated by the enhancement of autophagy after kidney injury. In a lipopolysaccharide-induced acute kidney injury rat model, Yang et al. demonstrated that DEX ameliorated the inflammatory response by reducing the NLRP3 inflammasome and inflammatory cytokines by enhancing autophagy via the AMPK/mTOR pathway. With the same acute kidney injury model, Zhao et al. found that DEX could protect against kidney injury by enhancing autophagy, thus removing damaged mitochondria and reducing oxidative stress and apoptosis through α2-AR and inhibition of the PI3K/AKT/mTOR pathway. Both animal models in Yang et al. and Zhao et al. 's studies were treated with DEX by intravenous injection with a dose of 30 μg/kg. The characteristics of the above relevant studies  reporting kidney injury are summarized in Table 2. The mechanisms of autophagy in the kidney-protective effects of DEX are illustrated in Fig. 3 (upper).

Lung injury
Acute lung injury, one of the serious forms of diffuse lung disease, has high morbidity and mortality and imposes a substantial health burden globally [75]. The common causes of acute lung injury include serious infection, burns, trauma, and shock. Fluid conservative strategy and lung-protective ventilation are the two certain supportive treatments to treat acute lung injury effectively. Recently, DEX has been suggested to exert protective effects on pulmonary functions in acute lung injury and ventilatorinduced lung injury [76]. Mechanistically, the lung-protective effects developed by DEX might be correlated to the autophagy-associated signaling pathways. To date, three studies [30,58,59] have reported the roles of autophagy in the action of DEX attenuating lung injury. All these studies indicated that the autophagic response was inhibited when treated with DEX in an animal model of lung injury. Zhang et al. [30] reported that preconditioning with DEX protected against lung injury in a dose-dependent manner by inhibiting autophagy, which might be associated with the upregulation of HIF-1α and downregulation of BNIP3 and BNIP3 L in a lung ischemia/reperfusion injury rat model. Ding et al. [58] showed that DEX protected against lipopolysaccharide-induced acute lung injury via reducing the inflammatory response and inhibiting autophagy-related proteins and the TLR4-NF-κB signaling pathway. Based on a toxic shock-induced lung injury rat model, Li et al. [59] found that DEX remarkably protected against lung injury by inhibiting autophagy and inflammation by decreasing the expression of pERK1/2 protein. The administration of DEX was the same in the above studies, viz. 50 μg/kg DEX intravenously. The characteristics of the relevant studies reporting lung injury are listed in Table 2, while the underlying mechanisms of autophagy in the protective effects of DEX are shown in Fig. 3 (lower).

Limitations and perspectives
To the best of the authors' knowledge, this is the first comprehensive review to summarize all the evidence of the crucial roles of autophagy in the action of DEX protecting against vital organ injuries. First, all the included studies listed in Tables 1 and 2 were either in vivo or in vitro experiments. The exact roles of autophagy in the human body under DEX treatment in organ injury have not been fully understood yet, which deserves further investigation. Second, the level of autophagy flux in the process of the DEX-mediated protective effect on organ injury is still controversial among different studies. Most of the included studies (17/24, 71%) demonstrated that the modulation of autophagy was inhibited during this process, but the remaining studies indicated that the autophagy level was promoted. This phenomenon is particularly observed in myocardial injury, with half of the included studies reporting inhibition and half reporting enhancement of the autophagy level. This inconsistency regarding the autophagy level might be due to the various timepoints monitored in different studies. Besides, autophagy may play a dual role in the protective effect against organ injury, which needs further investigation.

Conclusions
This review highlights the crucial roles of autophagy in the protective effect of DEX on multiple vital organs, including cerebral, myocardial, kidney, and lung injuries. The vast majority of the included studies have shown that the autophagy level is suppressed under treatment with DEX in organ injuries, but several studies suggested that the level of autophagy was dramatically increased after administration of DEX. Albeit not fully elucidated, the underlying mechanisms governing the roles of autophagy involve the antiapoptotic properties, inhibiting inflammatory response, removing damaged mitochondria, and reducing oxidative stress, which may be facilitated by the interaction with multiple associated proteins and signaling cascades. With the progress of extensive indepth studies, DEX-mediated autophagy will be fully understood to guide better clinical applications for organ protection.